I guess the difference for me is that BTRFS is dead easy (being mainline, as you said). The knock is reliability, but BTRFS has been rock solid every time I've used it, so from personal experience, it's a no-brainer.

Plus, I love snapper and the way subvolumes and snapshots are handled.

EDIT: I will add that I don't use any RAID. I use BTRFS for snapshots, cow and data integrity on my single drive desktops. I think that's where it ready shines. I don't think anyone should be using EXT4 anymore.